Frequency-modulated continuous-wave (FMCW) radar sensors operating in the millimeter-wave (mm-wave) region offer fine-resolution object detection and identification at short range. A linear frequency ramp with a minimum 6-GHz bandwidth is required to achieve a range resolution under 5 cm at 60 GHz. To synthesize an FMCW carrier an all-digital phase-locked loop (ADPLL) can be used which allows reconfigurability and permits a high degree of integration of the radio front-end with the digital baseband. MOS varactors are commonly used to tune DCOs in the low-GHz range. Their quality factor (Q), however, can be as low as ˜5 for 50 fF at 60 GHz and the varactor capacitance ratio, Cmax/Cmin, of ˜2 limits the DCO frequency tuning range. Moreover, a frequency tuning step lower than 1 MHz also cannot be achieved using varactor tuning.
All-digital phase-locked loops (ADPLLs) are currently used in numerous wireless applications in the low-gigahertz frequency range. Synthesizers at mm-wave frequencies, however, still rely upon analog-intensive PLL architectures due to the poor frequency resolution of conventional digital controlled oscillators (DCOs) tuned using switched MOS varactors. For an LC-tank resonating at 60-GHz band, typical tank inductance (L0) and capacitance (C0) values suitable for an oscillator implementation are 90 pH and 70 fF, respectively. An inductance smaller than ˜90 pH will have a lower Q-factor when implemented as a top-metal loop in most CMOS technologies. Thus, a fine-tuning step of 1 MHz for a 60-GHz carrier requires a capacitance change (ΔC0) as small as 2 aF (i.e.,
            f      0        =          1              2        ⁢        π        ⁢                                            L              0                        ⁢                          C              0                                            ,assuming
                              f          0                          C          0                    ≈                        Δ          ⁢                                          ⁢                      f            0                                    Δ          ⁢                                          ⁢                      C            0                                =          -                        f          0                          2          ⁢                      C            0                                ,thus
            Δ      ⁢                          ⁢              C        0              =                                        1            ⁢                                                  ⁢            MHz                                60            ⁢                                                  ⁢            GHz                          ·                  (                                    2              ·              70                        ⁢                                                  ⁢            fF                    )                    =              2.3        ⁢                                  ⁢        aF        ⁢                  )                      ,which is 1/25 the value of a minimum-sized NMOS varactor in 90-nm CMOS (i.e. 50 aF).
Furthermore, the Q-factor is as low as 5 for a 50-fF varactor at 60 GHz which severely affects the phase noise and jitter of a mm-wave DCO. Parasitic capacitance from interconnections contribute a significant fixed capacitance to the DCO tank which reduces the capacitive tuning ratio of the varactor (Cmax/Cmin) and results in a fractional tuning range smaller than 10% in practice. No prior art DCO operating above 50 GHz has been reported to date that has achieved 10% tuning range and sub-MHz tuning steps simultaneously.
There is thus a need for a millimeter wave digitally controlled oscillator (DCO) that exhibits wide tuning range (greater than 10%) while providing fine frequency resolution (less than 1 MHz).